As various electronic components used to make computer systems and peripherals have gotten increasingly smaller, so have the chassis in which they are placed. The most significant size decreases have been seen in more powerful computer designs such as servers.
Servers are now produced such that their chassis will fit into what is known as a hardware storage system or rack. These racks are employed to store computer systems with greater amounts of computing power while requiring a relatively small footprint. As a result of placing this computing power into racks, many peripherals commonly used in conjunction with servers have also been made to fit into racks.
To increase the effectiveness of the migration to rack mounted computer systems, standardization of chassis size and rack size has been implemented. The capacity of racks is often measured in height by the number of `U`s, where each `U` is generally equivalent to 1.75 inches. The standard internal height for "Tall" racks is typically 42U or 73.5 inches; "Short" racks are typically 24U or 42.0 inches in height. As computer and peripheral chassis are manufactured, their height dimensions are designed to occupy rack space in multiples of `U`, ranging upwards from 1U.
Each chassis in a rack is usually mounted onto a pair of slide assemblies allowing the components to be moved either forwards or backwards relative to the rack. Chassis displacement or movement relative to the rack is provided to allow the components to be replaced, inspected, maintained, or otherwise accessed. Chassis displacement is usually accomplished from the front of a rack by engaging the associated chassis and pulling the chassis forward. Rear displacement, however, has been performed in a variety of ways including engaging and pulling on the chassis's cabling or pulling on the chassis itself when accessible.
As anyone who has assembled a computer system or has seen the backside of a computer system can attest, access to the rear of a computer system is typically not obstacle free. Along with a myriad of cables plugging into the back of nearly all computer systems and peripherals, particularly rack mounted computer systems, the crowded nature of the rack contributes additional roadblocks to access and rearward displacement of rack mounted computer components.